There are two major current methods of rebuilding a very damaged tooth, restoration and crowning. Restoration essentially involves building up the damaged tooth by means of a restorative material in situ on the tooth; crowning involves forming a core on the tooth and forming, separately, a crown which is then cemented into place on the core.
Restorations are less time consuming and cheaper than crowning. A crown requires considerably more gross tooth reduction, and may have adverse effects on the periodontum. However, colour matching a restoration to a tooth is somewhat less reliable than colour matching a crown, because restoration is performed extempore, and the colour of a restoration may change gradually with the lapse of time. Restorations are also more liable to wear and to fracture than are crowns.
If a restoration becomes unsatisfactory, it can at some future time be reduced to form a core on which a crown is cemented provided that the original restoration provides adequate support and retention.
The use of dowels has become well established for the retention of core material for crowning. If the tooth is very damaged, the root canal is obturated, the coronal portion of the canal is enlarged, the dowel or post is inserted, core material is adapted to the dowel, and the core material and remaining tooth structure is reduced for crowning.
Various designs of dowel or post for this purpose are available. For example, there are screws of various lengths and diameters; custom castings made to approximate the internal size of the root canal; and parallel stainless steel knurled wired (which may have a vent channel up one side). A specific example of such a dowel is described in my earlier patent no GB 2 162 068 B. All these known dowels are normally fixed in the root canal by means of some form of luting agent (cement), through simple screwing into place has also been proposed.
There are many variations on the precise form of the dowel, which has to have one end firmly fixed in the root canal and in turn have either the restoration or the core firmly attached to its other end. To assist in the latter attachment, various forms of knurling and/or enlargements of the outer end of the dowel have been proposed. A specific is shown in the second embodiment of my earlier patent no GB 2 162 068 B noted above. In this patent is shown a dowel the top end of which has projecting lobes and a jagged top, and adds that the top end of the dowel can be made with a myriad of different shapes to help in retaining the core material to be attached to the dowel.
There is a wide variety of tooth sizes and several tooth types. It was stated in my earlier patent no GB 2 162 068 B noted above that "A set of dowels may be made in a convenient sequence of lengths and tip diameters. It is also possible to make the dowel with an angle or bend near the top end, for use in resetting" (i.e., retaining) "the crown of a projecting (buck) tooth." (Another use of a bent dowel would be to support core material or restoration material when the core or restoration diverges from the long axis of the root canal.) In addition to the various lengths and bends required, the enlargement at the top end of the dowel is also required in a variety of sizes, to allow for the different sizes of restoration or core which may be required. Hence a wide variety of combinations of dowel sizes and shapes may be required.
One object of the present invention is to provide dowel structures which can be used either for a direct crowning or for a restoration which can later be reduced to a core which is then crowned.